The incidence of invasive pneumococcal disease (IPD) changed markedly after the introduction in 2000 of a protein conjugate vaccine with capsular polysaccharides (PPS) of seven of the most common disease-causing serotypes (ST) of Streptococcus pneumoniae (pneumococcus) (PCV7). The most dramatic change was a major reduction in IPD with PCV-included STs due to herd protection, but this was accompanied by the emergence of non-vaccine STs and a marked rise in serotype 3 (ST3). ST3 is a leading cause of pneumonia and empyema in adults and children in the U.S. and globally and portends an independent, higher risk of death than other STs. Though there is a ST3 moiety, pneumococcal capsular polysaccharide 3 (PPS3) in the 23- valent PPS vaccine (PV23) used in adults, data on whether PV23 prevents pneumonia are conflicting and the ability of a new conjugate with PPS3, PCV13 to prevent ST3 is uncertain. Thus, there is an urgent need for a better understanding of antibody immunity to ST3 and to find better new ways to treat ST3 disease. The goal of this application is to identify and characterize human antibodies that protect against ST3. To accomplish this we will isolate human monoclonal antibodies (huMAbs) to PPS3 from PPS vaccine recipients by an innovative, single cell expression cloning approach and determine their efficacy in mouse models of ST3 colonization, pneumonia and sepsis as a function of their gene use and diversity, PPS3 epitope specificity, and functional activity against ST3 in vitro, taking the first steps in identifying therapeutic huMAbs for ST3. In addition, we wil use the epitopes and functional aspects of protective huMAbs as probes to interrogate PPS3 responses of PPS vaccine recipients. The huMAbs developed in this project will serve as candidate therapeutics for ST3 and the new scientific information it provides on antibody immunity to ST3 will make it possible to dissect PPS3 responses to vaccines in a way that has not been possible before and to identify novel, protective PPS3 antigens for ST3 vaccines. Thus, the project will have a major impact on clinical medicine by removing what have been formidable roadblocks in prevention of ST3 via novel therapeutic agents and on basic science via new insights into antibody immunity, ST3 pathogenesis and host-ST3 interaction.